Existing wavelength division multiplex (WDM) systems comprise individual transmitters with laser sources and data modulators which provide respective optical signals that are multiplexed together and transported via an optical fiber. Each individual transmitter has a laser source and a controller that measures and corrects the laser frequency and power. This controller, sometimes termed a wavelength locker, controls the resulting optical signal and effectively determines the long and short-term stability of the laser.
Where there is a need to increase the desired laser stability, to offset the laser frequency or to change optical channel spacing, then the individual transmitter must be replaced with one having the appropriate characteristics. In addition, since individual transmitters typically establish an operating frequency without knowledge of the exact frequency of an adjacent channel, the exact channel spacing uncertainty is equal to the uncertainty of the individual transmitter frequency multiplied by two. In addition, individual channel spacing can vary across the frequency band.
Multiple transmitters in WDM systems are followed by a channel multiplexer, which combines multiple fiber inputs into a single output fiber carrying all the channels at the same time, where each channel occupies a respective spectral region.
Since multiplexer channel spacing is matched to the channel spacing of the individual lasers, the manufacturing or operating tolerances may result in less than a perfect channel spacing match.